Method for burnishing iron and steel articles



Patented a; 8, 1940 UNITED STATES METHOD FOR BURBl'ISHING IRON AND STEEL ARTICLES Gerhard Zapt, Lubeck, Germany, assignor to Bernhard Berghaus, Berlin-Lankwitz,

many

Ger-

No Drawing. Application May 23, 1939, Serial No. 215,322. In Germany May 25, 1938 6 Claims.

For blackening steel and iron parts an immersion burnishing bath is used in the art which contains soda lye and sodium nitrite or sodium nitrate. The material being burnished is heated or boiled therein for 20-40 minutes at 135-145". A black covering layer having a certain protective efl'ect against rust is then formed on the iron. The process'has the drawback that the protective layer produced has only a low protective power towards corrosion and can be mechanically removed or worn away extremely easily. Furthermore, cast iron cannot be blackened by this process. On hardened and unhardened parts irregular colourings appear, and parts of alloyed steels, for example manganesesilicon steels, chrome nickel steels and tungsten steels, assume only a reddish-brown colour.

The drawbacks just referred to are removed by the present invention, which relates to a burnishing salt mixture and a burnishing proc- GSS'glIlIWhiCh the burnishing procedure is carried out "in a plurality of stages.

The parts are treated for 50-60 minutes in two or three successive burnishing baths of diiferent '25 composition, and thereby acquire a covering layer of extraordinarily high resistance to mechanical wear and very good stability towards corrosion.

If all that is required is a satisfactory blackening, even of alloyed material and cast iron, workingin two baths is sufficient. If on. the other hand high requirements are at the same time imposed on the mechanical wear strength and the corrosion protection afforded by the covering layer, then three baths must be employed.

' 5 For the preparation of the burnishing baths the burnishing mixture for burnishing stage baths described below is used with advantage:

' (1) For bath I a mixture of 73-77%, preferably 75%, sodium hydroxide (NaOH), 14--22%, preferably 16.5%, di-sodium phosphate (Na2HPO4+ 121-120) the mixture the parts are burnished by immersion for about 15 minutes at 125-1352 preferably at 128.

(2) For the second treatment stage a mixture is used consisting of 50 70% sodium hydroxide (NaOH), preferably NaOH, and

- dissolved up with 40 parts by weight of water to a 5 burnishing liquor in which the iron parts are burnished by immersion for about 15 minutes at 130-138", preferably 133.

(3) -For the third treatment stage finally a mixture is used consisting of 55-70%, preferably 10 60%, of sodium hydroxide (NaOH), 8-12%, pref erably 10%, of di-sodium phosphate (Na2HPO4+ 121-120) 10-18%, preferably 14%, of trisodium phosphate 5 and 2.5-4%, preferably 3%, of sodium nitrite (NaNOz) In order to improve the action up to 4% of sodium carbonate and 0.5% of sodium iodate may be added. 'For preparing the burnishing bath 60 parts by weight of this salt mix- 20 ture are mixed with 40 parts by weight of water. The iron parts are heated in this bath, at 145-160, preferably at 150, with advantage for about 30 minutes.

When working according to the process just 25 described layers can be produced showing about the four-fold resistance to wearand twice the protection to corrosion as compared with those produced according to the immersion burnishing processes known hitherto.

What I claim is:

1. A process for burnishing steel and iron articles comprising immersing the parts first in a bath containing 73 to 77% sodium hydroxide, 14 to 22% disodium phosphate and 3 to 7% so- 35 dium nitrite, then in a bath containing '50 to sodium hydroxide, and 30 to 50% sodium nitrite and finally in a bath containing 55 to 70% sodium hydroxide, 8 to 12% disodium phosphate, 10 to 18% trisodium phosphate and 2.5 to 40 4% sodium nitrite.

2. A process for burnishing steel and iron articles comprising immersing the parts first in a bath containing sodium hydroxide, 16.5% disodium phosphate and 5% sodium nitrite, then in a bath containing 60% sodium hydroxide, and 40% sodium nitrite and finally in a bath containing 60% sodium hydroxide, 10% disodium phos phate, 14% trisodium phosphate and 3% sodium nitrite. 60

3. A process as claimed in claim 1 whereas the first bath also contains 0.5% sodium iodate, the second bath contains 0.5% potassium iodide and the third bath contains 4% sodium carbonate and 0.5% sodium iodate.

4. A process as claimed in claim 1 wherein the temperatures of the three baths are 128 0., temperature of the first bath is between 125 133 C., and 150 C., respectively. and 135 C., the temperature of the second bath 6. A process as claimed inclaim 1 wherein the between 130 and 138 C., and the temperature of parts are immersed in the first bath for 15 5 the third bathbetween 145 and 160 C. minutes, in the second bath for '15 minutes and 5 5. A process as claimed in claim 2 wherein the in the third bath for 30 minutes.

GERHARD ZAPF. 

